Catalytic converter component and process for its manufacture

ABSTRACT

In a process for manufacturing a catalytic converter component, a ceramic unit is used that has been prepared by extruding green ceramic product through a die to form an extrusion having a honeycomb substrate structure in which tubular passages extend along the extrusion, the passages bounded by walls dividing adjacent passages from one another. The unit is obtained by cutting off a length of the extrusion and curing and firing it. The process further comprises flowing insulation material from one end of the unit into selected ones of the elongate passages, the insulating material then being cured by microwave irradiation. The passages are selected so that the cured insulation material forms an internal thermal insulating barrier between a core zone of the unit and a radially outer zone of the unit.

CROSS REFERENCE TO RELATED PATENTS

The present U.S. Utility Patent Application claims priority pursuant to35 U.S.C. §119(e) to the following U.S. Provisional Patent Applicationswhich are hereby incorporated herein by reference in their entirety andmade part of the present U.S. Utility Patent Application for allpurposes:

1. U.S. Provisional Application Ser. No. 61/692,732 entitled “Acatalytic converter component and process for its manufacture,” filedAug. 24, 2012, pending.

2. U.S. Provisional Application Ser. No. 61/733,949, entitled “Acatalytic converter component and process for its manufacture,” filedDec. 6, 2012, pending.

FIELD OF THE INVENTION

This invention relates to processes for the manufacture of catalyticconverter components for motor vehicles and to converter components madeby such processes.

BACKGROUND

The U.S. Department of Transportation (DOT) and the U.S. EnvironmentalProtection Agency (EPA) have established U.S. federal rules that setnational greenhouse gas emission standards. Beginning with 2012 modelyear vehicles, automobile manufacturers are required to reducefleet-wide greenhouse gas emissions by approximately five percent everyyear. Included in the requirements, for example, the new standardsdecree that new passenger cars, light-duty trucks, and medium-dutypassenger vehicles must have an estimated combined average emissionslevel no greater than 250 grams of carbon dioxide (CO₂) per mile invehicle model year 2016.

Catalytic converters are used in internal combustion engines to reducenoxious exhaust emissions arising when fuel is burned as part of thecombustion cycle. Significant among such emissions are carbon monoxideand nitric oxide. These gases are dangerous to health but can beconverted to less noxious gases by oxidation respectively to carbondioxide and nitrogen/oxygen. Other noxious gaseous emission products,including unburned hydrocarbons, can also be converted either byoxidation or reduction to less noxious forms. The conversion processescan be effected or accelerated if they are performed at high temperatureand in the presence of a suitable catalyst being matched to theparticular noxious emission gas that is to be processed and converted toa benign gaseous form. For example, typical catalysts for the conversionof carbon monoxide to carbon dioxide are finely divided platinum andpalladium, while a typical catalyst for the conversion of nitric oxideto nitrogen and oxygen is finely divided rhodium.

A catalytic converter may take any of a number of forms. Typical ofthese is a cylindrical substrate of ceramic material generally called abrick. The brick has a honeycomb structure in which a number of smallarea passages or cells extend the length of the brick, the passagesbeing separated by walls. There are typically from 400 to 900 cells persquare inch of cross-sectional area of the substrate unit and the wallsare typically in the range 0.006 to 0.008 inches in thickness. Theceramic substrate is formed in an extrusion process in which greenceramic material is extruded through an appropriately shaped die andunits are cut successively from the extrusion, the units being then cutinto bricks which are shorter than a unit. The areal shape of thepassages may be whatever is convenient for contributing to the overallstrength of the brick while presenting a large contact area at which theflowing exhaust gases can interact with a hot catalyst.

The interiors of the passages in the bricks are wash-coated with a layerof the particular catalyst material. The wash-coating is prepared bygenerating a suspension of the finely divided catalyst in a ceramicpaste or slurry, the ceramic slurry being to obtain adhesion of thewash-coated layer to the walls of the ceramic substrate. As analternative to wash-coating to place catalyst materials on the substratesurfaces, the substrate material itself may contain a catalyst componentso that that the extrusion presents catalyst material at the internalsurfaces bounding the substrate passages or cells.

A catalytic converter may have a series of such bricks, each having adifferent catalyst layer depending on the particular noxious emission tobe neutralized. Catalytic converter bricks may be made of materialsother than fired ceramic, such as stainless steel. In addition, ceramicsubstrates may have different forms of honeycombed passages than thosedescribed above. For example, substrate cells can be hexagonal ortriangular in section. In addition, if desired for optimizing strengthand low thermal capacity or for other purposes, some of the extrudedhoneycomb walls can be formed so as to be thicker than other of thewalls or formed so that there is some variety in the shape and size ofhoneycomb cells. Junctions between adjacent interior cell walls can besharp angled or can present curved profiles.

The wash-coated ceramic honeycomb brick is wrapped in a ceramic fibreexpansion blanket. A stamped metal casing transitions between the partsof the exhaust pipe fore and aft of the catalytic converter andencompasses the blanket wrapped brick. The casing is made up of twoparts which are welded to seal the brick in place. The expansion blanketprovides a buffer between the casing and the brick to accommodate theirdissimilar thermal expansion coefficients. The sheet metal casingexpands many times more than the ceramic at a given temperature increaseand if the two materials were bonded together or in direct contact witheach other, destructive stresses would be experienced at the interfaceof the two materials. The blanket also dampens vibrations from theexhaust system that might otherwise damage the brittle ceramic.

In use, the encased bricks are mounted in the vehicle exhaust line toreceive exhaust gases from the engine and to pass them to the vehicletail pipe. The passage of exhaust gases through the catalytic converterheats the brick to promote catalyst activated processes where theflowing gases contact the catalyst layer. Especially when the vehicleengine is being run at optimal operating temperature and when there issubstantial throughput of exhaust gases, such converters operatesubstantially to reduce the presence of noxious gaseous emissionsentering the atmosphere. It is known, however, that such converters haveshortcomings at start-up when the interior of the brick is not at hightemperature and during idling which may occur frequently during citydriving or when stopping for a coffee at Tim Hortons. The radialtransmission of heat in this and other forms of catalytic converteroccurs by a combination of convection, conduction and radiation. Thevarious heating mechanism have different effects at different convertertemperatures. In particular, at low temperatures before the converterhas reached optimal operating temperature, heat transfer ispredominantly by convection of gases and by conduction along and throughthe interconnected ceramic walls. At normal operating temperature, heattransfer is predominantly by radiation generally from the core of theconverter towards its periphery.

U.S. Pat. No. 8,309,032 (Plati et al.), which is herein incorporated byreference in its entirety, describes a particular form of catalyticconverter component for use in an exhaust system of an internalcombustion engine. The component includes a housing having a gas inletand a gas outlet, and catalytic substrate material filling the housing.The substrate material is divided into zones that are separated from oneanother by an insulating barrier, the zones defining flow passagesconnecting the inlet and outlet for the passage of exhaust gases. Incertain operating regimes, this configuration results in a reduction inheat transfer between a core zone and a surrounding zone of thecomponent. Thus, at start up as a majority of relatively cool gases flowthough a central part of the converter brick, heat tending to transferradially outwardly from the core zone by convection and conduction isinhibited by the presence of the insulating barrier. The core of theconverter component thus heats more rapidly from a cold start comparedwith a conventional catalytic converter without the thermal insulatingbarrier. When the converter component is operating at an optimaloperating temperature, any heat transfer is predominantly by radiationwhich is affected by the insulating barrier to a much reduced extent.

A reverse effect occurs when the engine is at its optimal operatingtemperature, but the vehicle experiences a period of idling. At thispoint, the reduced level of exhaust gases passing into the converterstart to localize along the converter core and also start to cool theconverter down. The presence of the thermal insulating barrier meansthat the temporary cooling effect is localized in the core zone and isnot rapidly or significantly transferred to the radially outer zone ofthe ceramic brick.

The Plati et al. structure promises significant improvements in loweringemissions and improving fuel mileage and precious metal catalystsavings. In particular, it means that ceramic substrates having of theorder of 400 cells per square inch can achieve low emissions which, inthe absence of the thermal insulation barrier, would require a substratehaving of the order of 900 cells per square inch loaded with preciousmetal catalyst. However, the placement of an insulating barrier within acatalytic converter component presents difficult manufacturing issues.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a processfor manufacturing a component for a catalytic converter comprisingtaking a ceramic unit prepared by extruding green ceramic productthrough a die to form an extrusion having a honeycomb substratestructure having a plurality of tubular passages extending along thelength of the extrusion, the passages bounded by walls dividing adjacentpassages from one another, the unit being obtained by cutting off alength of the extrusion and curing and firing the length of extrusion,the process further comprising flowing insulation material from an endof the unit to form an internal thermally insulating barrier between acore zone of the unit and a radially outer zone of the unit, and curingthe flowed insulating material using microwave radiation. The microwaveradiation can be generated by at least one microwave generator arrangedaround the ceramic unit with the microwave radiation penetrating theinsulation material from both sides of the barrier layer. During themicrowave curing, insulation material expanding from the ends of theunit can be cooled and removed by machining.

Preferably, the flowing insulation material from the end of the unit issuch as to at least partially fill selected ones of the passages withthe insulation material. Alternatively, a part of the honeycombstructure is cut way and the flowing of the insulation material from theend of the unit is such as to at least partially fill the site of thecut away part of the honeycomb structure with the insulation material.The insulation material can be injected into the selected passages as apaste-like mixture of glass fibres, ceramic slurry, binder and water.Alternatively, the insulation material is a powder mix of glass fibres,ceramic, and binder which flows into the selected passages under gravitywith vibration being applied to encourage the insulation flow and toencourage compaction of the powder in the selected passages. Preferably,the curing parameters are set so that following curing, the curedinsulating material is predominantly a matrix of silica containing porespredominantly in the range 1.5 millimetres to 0.3 millimetres across.

According to an alternative aspect of the invention, the walls betweenselected passages are cut away to leave a chamber, the insulationmaterial is injected or poured into the chamber before curing and thenis cured by microwave curing.

Alternatively, the ceramic extrusion is extruded so as to leave sitesfor the thermal insulation barrier devoid of the honeycomb substratestructure, and the insulation material is injected from an end of theunit into the sites before being microwave cured.

Preferably, the extrusion has a generally circular cross-section and thepassages are of square section and form a regular array. The substratematerial can be a form of aluminum magnesium silicate, A₁₄Mg₂Si₅O₁₈.

According to another aspect of the invention, there is provided acomponent for a catalytic converter comprising a unit of cured ceramicextrusion having a honeycomb substrate structure in which a plurality ofpassages extends along the unit, the passages bounded by walls dividingadjacent passages from one another, selected ones of the passages filledwith microwave cured flowed insulation material, the microwave curedflowed insulation material forming an internal thermal insulationbarrier between a core zone of the unit and a radially outer zone. Themicrowave cured flowed insulation material can be cured flowed masticinsulation or cured flowed powder insulation.

BRIEF DESCRIPTION OF THE DRAWINGS

For simplicity and clarity of illustration, elements illustrated in thefollowing figures are not drawn to common scale. For example, thedimensions of some of the elements are exaggerated relative to otherelements for clarity. Advantages, features and characteristics of thepresent invention, as well as methods, operation and functions ofrelated elements of structure, and the combinations of parts andeconomies of manufacture, will become apparent upon consideration of thefollowing description and claims with reference to the accompanyingdrawings, all of which form a part of the specification, wherein likereference numerals designate corresponding parts in the various figures,and wherein:

FIG. 1 is a perspective view showing a ceramic substrate unit formingpart of a ceramic extrusion;

FIG. 2 is a longitudinal section through a known prior art arrangementof a catalytic converter;

FIG. 3 is a sectional view through an insulation injection station usedin a process according to an embodiment of the invention;

FIGS. 4 and 5 are end and sectional views respectively of an insulationinjection die for use in a process according to an embodiment of theinvention;

FIG. 6 is a perspective view showing a ceramic substrate unit accordingto an embodiment of the invention;

FIG. 7 is a longitudinal section through a catalytic converter accordingto an embodiment of the invention;

FIG. 8 is a detail to a larger scale of the structure illustrated inFIG. 7;

FIG. 9 is a detail to the larger scale of a structure similar to thestructure illustrated in FIG. 7 but constructed by an alternativeprocess according to an embodiment of the invention;

FIG. 10 is a detail to a larger scale of part of the structure of FIG.6;

FIG. 11 is a perspective view showing a ceramic substrate unit accordingto an embodiment of the invention; and

FIG. 12 is a longitudinal section through a catalytic converteraccording to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING THE PRESENTLY PREFERREDEMBODIMENTS

Referring in detail to FIG. 1, there is shown in perspective,part-outline view a length of extruded ceramic substrate commonly termeda unit 10, the unit consisting of two shorter lengths, termed bricks 12that are for use as catalytic converter components. Each brick 12 has anumber of tubular passages 14 extending throughout its length, thetubular passages divided by walls 16. The representation of the bricks12 in FIG. 1 are not to scale and, across the area of a brick, there arein the range of 400 to 900 cells per square inch with wall thicknessesbeing of the order of 0.006 to 0.008 inches. The length of a brick 12 isgenerally of the order of 3 inches but a brick may be shorter dependingon how it is to be loaded with catalyst material as will be describedpresently. Such brick designs are well-known in the catalytic converterart. Extrusion dies (not shown) for the ceramic honeycomb extrusion aredesigned to take a single flow source from an extruder and split theflow into a substrate having a cell density in the required range of 400to 900 cells per square inch. Because friction generated in the dieproduces high back pressures on the back side of the die, the ceramicmaterial is extruded in a relatively soft green state. A suitableceramic material for the substrate is cordierite, which is a form ofaluminum magnesium silicate A₁₄Mg₂Si₅O₁₈. It has a low thermal expansioncoefficient, moderately high strength, and low cost which makes it afavored choice for the substrate component of catalytic convertercomponents. Alternative substrate materials can also be used. Forexample, silicon carbide may be advantageously used for certain hightemperature or diesel applications. The substrates are produced byextruding paste-form, green ceramic paste through an extrusion die.Units are cut from the extrusion as it emerges from the die. The unitsare then kiln dried and fired to a sintering temperature of the order of1300 degrees C.

As is also known in the art, the bricks are immersed in a wash-coatcontaining a base material suitable for ensuring adherent coating ontothe cured ceramic material and entrained catalyst material for use inpromoting specific chemical reactions. The outside of the ceramicsubstrate unit is masked to inhibit the catalytic layer from coating theoutside of the unit. Examples of such catalyst materials are platinumand palladium which are catalysts effective in converting carbonmonoxide and oxygen to carbon dioxide, and rhodium which is a catalystsuitable for converting nitric oxide to nitrogen and oxygen. Othercatalysts are known which promote high temperature oxidation orreduction of other gaseous materials. Following drying of the wash-coat,the brick is mounted within a casing 18 and expansion blanket 19 asshown in FIG. 2.

The casing 18 includes a front end adaptor section 20 for fixing to anupstream part of a vehicle exhaust pipe (not shown) and a back endadaptor section 22 for fixing to a downstream part of the exhaust pipe.The unit shown in FIG. 2 has two bricks 12 mounted in series, the brickshaving catalytic coatings which are different from one another so as topromote or accelerate different reactions within gaseous exhaustemissions. Thus, a first brick might have entrained platinum andpalladium catalysts for promoting high temperature conversion of carbonmonoxide to carbon dioxide. The second brick might have entrainedrhodium catalyst for promoting high temperature conversion of oxides ofnitrogen to nitrogen and oxygen. Additional bricks may be mounted in thecasing to promote yet other reactions. In addition, the bricks may beseparated and have their own casing.

Referring to FIGS. 3 to 5, in a process according to one embodiment ofthe invention, insulating material 24 is injected upwardly into a unit10 of the ceramic extrusion after it has been cut from the extrudedstock and after it has been cured and fired. The insulation material 24is injected into selected ones of the elongate passages 14 so that, asshown in FIGS. 6 and 7, the selected cells 14 are filled throughouttheir lengths. The passages or cells 14 are selected so that, once theinsulation material 24 is cured, it forms a thermal boundary layer 26between a core zone 28 and a peripheral zone 30 in the catalyticconverter brick 12.

Referring back to process FIGS. 4 and 5, in one process example, aninjection head 32 is made of stainless steel and has an externaldiameter of 3.66 inches matched to the 3.66 inch diameter of thesubstrate. The injection head 32 has an annular injection aperture 34having an internal diameter of 2.5 inches and an outer diameter of 2.8inches to produce an annular slot width of 0.15 inches. Alternativeforms and dimensions of injection head 32 are used for differentsubstrate cross sectional areas and shapes. In addition, alternativering aperture positions and widths can be adopted to obtain adifferently positioned thermally insulating boundary layer 26 or adifferent thickness of layer. Other suitable materials can be used forthe injection head provided that they are dimensionally stable over timeand when subjected to temperature variation, and provided also that thematerial of the injection head 32 (or at least the interior of theinjection path within the die) does not react with the insulationmaterial or adhere to it.

Robotically controlled handling equipment is used to move and clamp theinjection head 32 and the substrate unit 10 in a precise registration sothat selected cells 14 of the substrate are optimally aligned with thering aperture 34. Photoelectric sensors having light emitters anddetectors at opposite ends of the substrate are used to direct supercollimated light along the passages during the registration step. Thelevel and location of detected light are used to develop control signalsto the handling equipment to achieve fine adjustment of an initialregistration position of the injection head 32 relative to the ceramicsubstrate unit 10 so as to obtain the cleanest possible division betweencells 14 that are to be filled with injected insulation 24 and cellsthat are, to be free of insulation. This is supplemented by a machinecut gasket layer 36 at the injection face of the injection head. Thegasket layer has a ring shaped aperture that generally matches the ringaperture of the injection head but, as shown in FIG. 10, has a perimeterwhich is specifically matched to the boundaries of the cells to befilled. The gasket also accommodates any excursion from planarity of theend surface of the ceramic substrate into which insulation is to beinjected.

By directly injecting the honeycomb cells or passages 14, the structuralintegrity of the ceramic substrate is minimally affected. The small cellsize (as shown, 0.04×0.04×6 inches in the 400 cells per square inchsubstrate), is quite difficult to inject over the full 6 inch length ofa ceramic unit 10. Use of the low viscosity mastic insulation material24 enables the insulation to flow the full 6 inches length of the unitat a relatively low injection pressure of the order of 650 psi. However,through a combination of surface tension, viscosity, adhesion propertiesof the insulation, and porosity and surface roughness properties of thesubstrate ceramic, the insulation does not flow out of the cells onceinjected.

As previously mentioned, to achieve a clean injection surface, aprecision gasket layer 36 is present at the injection face of theinjection head 32 as shown in FIG. 3. The injection head and thesubstrate are mounted so that the head is pressed against the substrateunit to squeeze the gasket 36 so that it seals against the substrateunit to prevent leakage of insulation 24 into cells 14 adjacent thoseselected for injection. To achieve a clean surface at the other end ofthe substrate, either the area of non-selected cells is temporarilymasked off as shown at 38 or a similar gasket arrangement is used on thesubstrate exit end.

The insulation material in its injectable form is an essentiallyhomogeneous, very low viscosity masticated mixture of glass fibres, clayslurry, polymer binders and water. The low viscosity mastic insulationis readily injectable and exhibits Newtonian fluid properties. Themastic retains its shape upon dispensing and so remains within theselected tubular passages upon injection. The low viscosity allows themastic insulation material to be injected into all corners of thehoneycomb cellular space and the adhesion properties of the bindercomponent mean that it strongly adheres to the ceramic surfaces evenwith its low viscosity. For small cell sizes of the order of 900cells/passages per square inch, frictional resistance to the injectedinsulation flow is high and corresponding adjustment of componentcontents is required to modify the rheology of the insulation to enableeffective injection.

The insulation material has a maximum service temperature of the orderof 1200 degrees Centigrade and so is not damaged at the normal upperoperating temperature of conventional catalytic converters which is ofthe order of 1000 degrees Centigrade. The insulation material is durableand, while not as strong as the ceramic substrate, is well able tosurvive the combination of heat and vibration produced in regularoperation of an upstream internal combustion engine. The insulation isrelatively inert and, by its fibrous nature, resists thermal shocking.Once cured, the insulation forms a network of interlaced glass fibresthat are bonded together. The fibres are flexible and allow theinsulation to readily expand and contract upon thermal cycling withoutcreating failures or defects. The insulation material is engineered forvery low thermal conductivity to match the slightly higher thermalconductivity of the ceramic. While reasonably close in performance tothe surrounding ceramic, the mastic insulation material is designed torelax relative to the ceramic substrate to allow the insulation to bendand flex with the ceramic substrate but still retain its shape and notdegrade over time and temperature. The binder materials provide goodadhesion between the mastic and ceramic, this being enhanced by the highinterfacial surface area between the insulation and ceramic as afunction of the enclosed volume of insulation material. The solid bondbetween the mastic and ceramic produces an integral composite structure.

The substrate core diameter, i.e. the diameter of the core cylindricalzone 28 inside the thermal insulation boundary layer 26, is matched tothe exhaust pipe diameter. In one example, the core zone 38 has adiameter of 2.5 inches for automobile converter units mounted close tothe engines. In another example, where converter units are locatedunderneath the vehicle and at an appreciable distance from the engine,the core diameter is of the order of 2.75 inches. The majority ofexhaust gas flow through the converter is through the core zone 28 withthe proportion of that flow through the center being directly related tothe inlet exhaust diameter.

While in the preferred embodiment, the illustrated ceramic substrateunit 10 has a circular section and square cells, it will be appreciatedthat other substrate sectional shapes are possible such as rectangular,elliptic, oval, etc., for substrates having reduced ground clearancerequirements, and other cell sectional shapes are possible such ashexagonal, triangular, circular, etc.

The wash-coating is typically a single step process with the substratebeing dipped into a tank containing a precious metal slurry made up ofplatinum group metals (PGM), such as palladium, platinum and rhodium, ina clay suspension. The clay suspension carries the PGMs and provides abonding surface between the PGMs and the ceramic substrate. Substratelengths may be dipped multiple times to increase the PGM loading. Thewash-coated parts are heated to 650 degrees Centigrade to cure the claysuspension. Substrate units 10 typically of the order of 6 inches inlength are dipped in the catalyst solution and are then cut into smallerbricks 12.

The mastic insulation material used in the production of the bricks 12is microwave cured after being injected. The curing process is designedto heat the injected brick beyond the curing temperature of theinsulation: 100 degrees C. for the exemplary mastic insulation. Thistemperature is needed in order to eliminate water and liquid binders andto activate a polymerization process which converts the masticinsulation from a paste to a solid. The curing dynamics are set so thatthe insulation material as it solidifies forms a matrix ofinterconnected pores which contain the vaporizing water, the pores beingof the order of 1.5 millimetres and less across. During curing, watervapour flows through the matrix and through the walls of the ceramichoneycomb walls before escaping from the unit. By appropriately tuningcuring parameters, including water content of the injected insulationand speed of curing, a pore size and distribution is obtained whichoffers the best optimization for structural strength and foreffectiveness of thermal insulating barrier.

Once cured, the insulation is structurally stable, does not revert backto a paste form in the presence of water, and is essentially unaffectedby a temperature of the order of 1000 degrees C. or by temperaturecycling between room temperature and 1000 degrees C. As the water andliquid binder gases off, the ceramic unit stops absorbing microwaveradiation and so stops heating up with the temperature not gettingsignificantly hotter than the 100 degrees C. level required for curing.The previously fired honeycomb ceramic is largely transparent to themicrowave radiation.

It is noteworthy that microwave curing is faster and more efficient thanconventional convection curing (air curing or oven curing). Microwavecuring of the injected region takes less than 10 minutes in a 1 kilowattmicrowave unit. Faster curing can be achieved using one or moreindustrial grade microwave generators with such generators being used tocure several units simultaneously. However, the rate of curing must notbe so fast that a rapid vaporization of water and a relatively slowersolidification of the insulation and binders cause loss of integrity inthe structure; i.e. not so fast that the vaporizing water is explosivelyejected causing loss of homogeneity in the cured insulation or damage tothe honeycomb ceramic. In comparison, oven curing the same injectedbrick structure takes of the order of 3 hours in a 910 kilowatt oven.This represents a significant saving in time and energy costs comparedwith conventional oven curing. In this respect, a disadvantage of ovencuring is that the mastic insulation is entrained inside the ceramichoneycomb matrix which must be heated to the curing temperature in orderto transfer the heat by conduction to the insulation. The honeycombceramic brick is highly insulating owing to the nature of the materialitself and the cellular geometry which together make for inhibited heattransfer resulting in long curing times and wasted heat. In the case ofmicrowave curing, the ceramic honeycomb matrix is virtually invisible tomicrowave so that the microwave energy goes straight to the insulationto effect the curing process. The insulation quickly heats up and cureswith very little heat loss to the ceramic honeycomb. The thincylindrical shell sections lend themselves particularly to microwavecuring. Thus, microwave curing has a maximum penetration depth which isnot far below the surface of the mastic insulation paste—about 2millimetres or 0.080″. The microwaves bombard the thin shell sectionsfrom both sides, effectively doubling the penetration depth to about 4millimetres or 0.160″. The injected thermal management zone is about0.150″ thick which is within the 0.160″ penetration depth.

The mastic insulation expands slightly with microwave curing therebyreducing the density of the insulation material but increasing itsthermal insulating capacity. Any cured insulation projecting from theend surfaces of the ceramic unit is removed. In the finished product,the pores constitute air pockets that give the insulation its insulatingvalue and produce its low thermal conductivity. The pores are isolatedfrom one another and have a high insulating value in comparison withdense, solid material because heat that transfers to the insulation mustcircumvent the pores to propagate through the insulation layer. The poredistribution results in relatively circuitous heat transfer routes sothat the effective travel distance for heat in transferring from oneside of the thermal barrier to the other is greater in the porousmaterial compared to a solid material.

Heat transfer within the insulating barrier is by a combination ofconvection, radiation and conduction. In the first case, convectionoccurs by movement of air molecules in each of the pores as convectionof air in a pore close to the hot ceramic causes the pore to heat up.The pore then transfers heat to adjacent pores and so on through thepore matrix. Radiation heat transfer becomes significant when theconverter is very hot. Here, a hot face of a pore radiates to theopposite face of the pore regardless of air movement. The presence ofthe porous insulation reduces the rate at which heat is radiated acrosseach passage in comparison with an empty passage. Thus, with openpassages, the cells are oriented in a given direction (direction of heattransfer) and there is minimal surface area for intense radiation(energy per unit area). In contrast, porous insulation is randomlyoriented (heat travels in all directions) and has a large surface area,meaning lower radiation intensity. Conductive heat transfer also takesplace but is less of a factor in the thermally insulation character ofthe barrier. Typically, the thermal conductivity of the ceramic is 4W/mK and of the insulation is 0.25 W/mK.

The microwave curing has other benefits compared with conventionalcuring methods such as air curing and oven curing. Thus, air curingproduces a contraction of the mastic insulation material which meansthat, after curing, the barrier cavity is not completely filled to theends of the unit. This means that there will be some reduction ininsulating capacity unless an additional process step is taken to fillthe barrier cavities to the unit ends.

Unlike microwave curing, neither air curing nor oven curing produces auniform pore size or pore distribution. The nature of the pores isimportant to the structural integrity of the unit and to its thermalinsulating properties. During microwave curing, the pores are formed byvaporization of liquid components in the mastic insulation, with thenature and size of the pores that are formed being dependent on curingprocess parameters including the curing rate of the insulation in theceramic unit. The liquid components are water and organic binders, thelatter being essentially polymer chains in suspension. Upon heating, themajor liquid component, water, vaporizes while the polymer componentremains. When the water vaporizes, water droplets undergo expansion asthey change state. Owing to the mastic nature of the insulation—it istacky and has high surface tension—vapour bubbles are trapped in thesolidifying insulation so as to produce the pores. Curing parameters,chiefly the water content and the rate of curing, are selected to createuniform pore size and distribution.

In contrast, air or oven curing are slow procedures with the masticinsulation remaining liquid for a long time which allows it to flow andcoalesce. If such conventional curing processes are used in thisapplication, the insulation tends to migrate towards the walls of theceramic matrix. Consequently, initially formed small pores become largepores which may span the full width and a substantial part of the lengthof the thermal barrier. While large pores offer high thermal insulation,they are not good for structural integrity because it may mean that thetwo faces of the thermal barrier are not physically bonded together.Slow heat transfer also means that pores are produced at the ends of theunit or brick before they are formed over the middle region. This, inturn, results in non-uniform pore size and pore distribution, with thepores at the centre of the cavity being smaller and higher in numberthan at the ends of the cavity where pores are fewer but larger. Smallpore size means denser material and lower insulating capacity. Largepore size may result in lower structural integrity. Microwave curingheats the insulation so fast that coalescing is not an issue. The masticinsulation heats and cures resulting in small pores of uniform size anddistribution.

A thermal barrier cannot be effectively injected and fired during themanufacture of the ceramic honeycomb units because the honeycombstructure is extruded as softened clay. Consequently, injectinginsulation, even at very low pressure into the honeycomb causes thewalls to deform with the insulation intruding into and blocking adjacentpassages of the honeycomb. Although the embodiment describes the curingof an insulating barrier injected into a conventional honeycomb ceramicunit, having a uniform matrix of cells across its cross section, it willbe appreciated that microwave curing can be used for ceramic units whichare formed in alternative processes. In one alternative, a conventionalhoneycomb unit is machined to cut slots into the matrix that form theoutline of the thermal barrier into which mastic insulation is injectedbefore curing. In another alternative, special dies are used to extrudethe ceramic honeycomb so that it has slots formed in the extrudate. Themastic insulation is then injected into the slots of the cured extrusionand the injected insulation material is microwave cured.

The injected insulating material is cured to drive off water from themastic insulation and to activate polymer binders in a polymerizationprocess that solidifies the mastic material at its injected position.The binders in the insulation polymerize in a one-way reaction so thatany subsequent wetting of the cured insulation does not return it to itslow viscosity state. While the curing process is effected at lowtemperature of the order of 650 degrees Centigrade, the resultingthermal boundary can survive automotive application temperatures of theorder of 1000 degrees Centigrade.

Because of the insulating nature of the ceramic, the mastic insulationinside the ceramic substrate unit 10 is not easily accessed for curingpurposes and therefore cannot be easily and rapidly cured usingconventional heating methods. The insulation within the ceramic units istherefore cured by microwave radiation. Microwave absorbent materials inthe insulation—water and liquid binder—are energized by the microwaveradiation to heat such materials by atomic vibration. The solidcomponents including the ceramic are essentially transparent to themicrowave energy. The microwave radiation penetrates from all sides ofthe unit 10 simultaneously curing the insulation in the boundary layerquickly and evenly. As the temperature rises through 100 degreesCentigrade, the water vaporizes and the binder materials undergopolymerization. The maximum microwave penetration depth in theinsulation material is about 0.08 inches. A suitable thermal barrierboundary layer is one or two cell widths in thickness corresponding in a400 cells per inch honeycomb structure to a thermal insulation boundarythickness of 0.04 inches for one cell or 0.08 inches for two cells.Thus, considering that curing radiation is directed from both sides ofthe boundary layer 26, the insulation thickness to be cured is less thanthe microwave penetration depth so ensuring curing throughout thethermal boundary layer. While curing of a unit is rapid—of the order of30 seconds—the driving out of water vapor and binder gases does notdevelop internal gaseous pressures sufficient to damage the curedinsulation. If there is any slight distension of insulation at the endsurfaces of the unit, it can be quickly machined away. In use as acatalytic converter, the high temperature of the bricks causes anyremaining organic binder to be driven off leaving a porous inorganicmatrix of glass fibres (of the order of 90% silica by weight) andceramic.

The binder polymerization produces a one-way reaction where thesolidified insulation cannot be converted back into a mastic state byadding water or other liquids. The ceramic substrate is virtuallyinvisible to microwave, as there are no absorbent materials present inthe ceramic. This makes for an efficient curing process as the microwaveenergy is selectively absorbed by injected insulation material. Themicrowave curing produces a regular pore structure which is desirablefor achieving high performance in terms of presenting a thermal barrierto heat transfer between the core zone 28 of the converter unit 10 and aradially outer zone 30 of the unit.

By injecting the insulation material down the tubular passages 14 of thesubstrate units 10, the basic initial ceramic honeycomb structure ismaintained, together with the properties inherent in that structure. Theceramic honeycomb is brittle so there is very little distortion of thehoneycomb walls 16. In particular, the resistance to lateral compressionof the basic structure is substantially maintained. It will be notedthat by using this particular method, there is no machining ofinsulation injection cavities into the honeycomb which, regardless ofthe accuracy of the machining process, might produce stresses andmicro-flaws that would weaken the ceramic structure at the machinedregions.

As an alternative to injecting mastic insulation into thecells/passages, a powder-based insulation is vibrated into the cells.The powder-based insulation material is similar in composition to themastic insulation having corresponding ingredients includingalumino-silicate powder, glass fibres, polymer binder, and water. Incontrast to the mastic insulation which is initially a high viscosityliquid, the powder is a granular solid. The mastic insulation has amoisture level of the order of 40% by weight or more before curing,whereas the powder insulation has a moisture level of less than 15%. Thepowder-based insulation exhibits almost no flow under the application ofpressure. Instead, high frequency vibration (3450 CPM or less,CPM—cycles per minute) is applied to fluidize the powder which thenflows under gravity into the honeycomb so as to fill the cells from thebottom up. Similar to the mastic, a template gasket can be used todefine the cells that are to be filled from those that are to remainempty. The gasket has openings that align with cells to befilled/blocked and blocks access to cells that are to remain open/clear.Additional vibration is used to encourage compacting of the powder afterit falls into the cells if gravity alone is not enough to ensure adesired level of fill density of the cells so as to achieve the desiredintegrity of the insulation after curing. Proper filling is achievedwhen large air pockets have been vibrated from the cells. Whereas tinyair pockets assist the insulating value, excessively large air pocketswould have an adverse effect on insulating value. Vibration helps removethe larger air pockets, with more powder insulation then filling theareas from which the larger air pockets are removed. Mechanical lockingof adjacent particles is desirable as the powder reaches full settlingin the uncured (or green) state as it means that there will becorresponding high strength of the insulation matrix obtained afterCuring is performed by microwave or convection oven heating to activatethe polymer binder in the powdered insulation. This locks the insulationto itself and to the cells walls. Curing is essential so that theinsulation does not separate from the catalytic converter substrateduring handling in assembly or after the converter is installed and isbeing used in an internal combustion engine.

Powder insulation particle size is important for proper fluidization andfilling of the honeycomb cells. A particle size less than 30 mesh (0.033inch diameter) is preferred but powder down to 100 mesh in size can becontemplated. In fact, a combination of large and small particle sizesmakes for good compaction as the small particles fill interstitial voidsbetween the larger particles. Particles larger than 30 mesh areundesirable as the particle size is on the order of the cell diameterand could block the cell opening preventing insulation from getting intothe cell, or become wedged within the cell and inhibit proper filling.

The embodiment of FIGS. 6 and 7 shows a single annular insulatingboundary layer 26 extending throughout the length of the substrate unit10. It will be appreciated that variations in this structure areenvisaged depending on desired thermal heat flow transfercharacteristics. For example, in one alternative, the insulating layer26 can be interrupted in its circumferential and/or longitudinal extentso that a thermal barrier exists at certain interface regions betweenthe core zone 28 of the component and the peripheral zone 30, but nothermal barrier is present at other parts of the interface. In anotheralternative, a plurality of thermally insulating layers can be injected;for example, as a concentric series. In yet another alternative,particularly for insulating wall forms adapted to a non-cylindricalsubstrate unit 10, a non-cylindrical thermal boundary layer 26 may beinjected.

In a manufacturing environment, the ceramic substrate units are movedfrom an unpacking center by an in-feed conveyor to a process site.Finished items are moved from the process site on an out-feed conveyorto a packing center. The process site has several process stationsincluding a first weighing station, a visual inspection station adaptedto view ceramic unit ends, an insulation injection station, a microwavecuring station, an output weighing station and an output visualinspection station. The substrate units are moved between stations usingrobotic pick heads with the stations incorporating placement referencemeans for accurate location of the blanks suited to the particularprocess step.

While the preferred embodiments illustrated use a ceramic substrate, theprinciple of the invention can be adopted with converter substrates madeof other materials such as stainless steel. In the case of stainlesssteel, before injection of insulation takes place, the substrate unitsare formed by roll crimping/corrugating a sheet of stainless steel andthen winding the sheet into a tight cylinder.

Although less preferred because of potential damage to the ceramicsubstrate, in an alternative aspect of the invention, one or morecavities for the boundary layer(s) is removed by machining away thewalls of selected ones of the cells 14 so that when the insulationmaterial is injected into the ceramic unit 10, it fills a barrier cavityas shown at the detail of FIG. 7 rather than, as shown in the FIG. 8detail associated with the process of FIGS. 3 to 5, forming aninterstitial matrix with the walls 16 of the selected cells 14. In themachining process, a computer numerical control (CNC) machining processis used with tungsten carbide cutters being directed from opposite endsof the unit to maximize linearity. The machining is implemented so as toleave islands of the ceramic structure intact so as to keep thesubstrate in one piece. A suitable insulation injection die for use withthis structure also has connecting ribs between several arcuate dieaperture elements so that the insulation material is not injected at theislands.

In another embodiment of the invention as illustrated in FIG. 12, some,but not all, of the length of the ceramic unit over the area of selectedpassages is filled with mastic insulation and then microwave cured. Asshown, the insulation is injected to occupy the input end of the unit,with rest of the unit length constituting an air gap or gaps. Theinsulation can alternatively be injected a short length into both endsof the unit. The remaining air gap or gaps offers satisfactory thermalinsulating properties, although not preferred in comparison with a unitfilled along its full length with the cured mastic insulation.

In an alternative embodiment of the invention as illustrated in FIG. 11,a number of cavities 40 for the insulation barrier layer are left as aresult of the process for producing the honeycomb structure extrusion.The cavities are separated from on another by radially extending walls42 which provide structural integrity to the extrusion. The extrusion isfired and then injected from one end with mastic insulation so that theinsulation fills the chambers and provides a barrier layer which isessentially a hollow cylinder. A suitable insulation injection die foruse with this structure has connecting ribs between several arcuate dieaperture elements with the ribs aligned with the walls 42. The masticinsulation is then microwave cured.

In the processes described and illustrated, the paste-form masticinsulation is injected from one end of the ceramic unit. It will beappreciated, however, that the mastic insulation can be injected fromboth ends of a unit. For example, specific passages of a unit can beinjected from one end while other passages of the unit are injected fromthe other end. In a further embodiment, pinholes are formed at thecenter of a unit to allow the escape of air form the passages and thenthe mastic insulation is injected from both ends of the unit.

In the process described and illustrated, a gasket is used between theinjection die and the ceramic unit to ensure that the mastic insulationis injected into the intended passages. As an alternative to a separategasket, the die material is formed of slightly deformable material suchas high density plastic or PTFE. Consequently, when the die is pressedagainst the end of the ceramic unit, the die undergoes some rudimentarydeformation sufficient to match the die to minor excursions fromplanarity of the end of the ceramic unit.

It will be seen that the process described with reference to FIG. 3 canbe effected on industry-standard ceramic substrates so as to affect thepresent-day supply chain sequence associated with such catalyticconverter units only to a minimal extent. Direct injection of insulationleaves a converter unit design very much like the standard substrateunit currently used in automobile manufacture.

What is claimed is:
 1. A process for manufacturing a component for acatalytic converter, comprising taking a ceramic unit prepared byextruding green ceramic product through a die to form an extrusionhaving a honeycomb substrate structure having a plurality of tubularpassages extending along the length of the extrusion, the passagesbounded by walls dividing adjacent passages from one another, the unitbeing obtained by cutting off a length of the extrusion and curing andfiring the length of extrusion, the process further comprising flowinginsulation material from an end of the unit to form an internalthermally insulating barrier between a core zone of the unit and aradially outer zone of the unit, and curing the flowed insulatingmaterial using microwave radiation.
 2. A process as claimed in claim 1,in which the flowing insulation material from the end of the unit issuch as to at least partially fill selected ones of the passages withthe insulation material.
 3. A process as claimed in claim 1, furthercomprising cutting away a part of the honeycomb structure, the flowinginsulation material from the end of the unit being such as to at leastpartially fill the site of the cut away part of the honeycomb structurewith the insulation material.
 4. A process as claimed in claim 2, inwhich the insulation material is injected into the selected passages asa paste-like mixture of glass fibres, ceramic slurry, binder and water.5. A process as claimed in claim 2, in which the insulation materialflows into the selected passages under gravity as a powder of glassfibres, ceramic, and binder.
 6. A process as claimed in claim 1, inwhich, following curing, the cured insulating material is predominantlya matrix of silica containing pores predominantly in the range 1.5millimetres to 0.3 millimetres across.
 7. A process as claimed in claim1, the microwave radiation directed from both sides of the barrierlayer.
 8. A process as claimed in claim 1, the insulation materialdirected through a die to obtain a generally cylindrical thermalinsulation barrier.
 9. A process as claimed in claim 1, furthercomprising using a gasket layer on at least one end of the unit to limitincursion of insulation material into non-selected passages.
 10. Aprocess as claimed in claim 1, further comprising incrementallyrelatively moving an extrusion die and the unit to optimize registrationof an exit aperture of the die to entrance openings of the selectedpassages.
 11. A component for a catalytic converter comprising a unit ofcured ceramic extrusion having a honeycomb substrate structure in whicha plurality of passages extends along the unit, the passages bounded bywalls dividing adjacent passages from one another, and an internalthermal insulation barrier between a core zone of the unit and aradially outer zone, the insulation material barrier composed ofmicrowave radiation cured insulation material.
 12. A component asclaimed in claim 11, the thermal barrier formed by at least part ofselected passages containing the microwave cured insulation material.13. A component as claimed in claim 12, the microwave cured insulationmaterial being cured from a past-like insulation material injected intothe selected passages.
 14. A component as claimed in claim 12, themicrowave cured insulation material being cured from a powder insulationmaterial gravity flowed into the selected passages.
 15. A component asclaimed in claim 12, in which the insulation material contains glassfibres, ceramic, and binder.
 16. A component as claimed in claim 12, inwhich, following microwave curing, the cured insulating material ispredominantly a matrix of silica containing pores predominantly in therange 1.5 millimetres to 0.3 millimetres across.
 17. A component asclaimed in claim 12, the component mounted within a casing, the casinghaving an inlet adaptor for entry of exhaust gases from an upstreamexhaust pipe section to one end of the component for transmission alongnon-selected ones of the passages, and an outlet adaptor for exit ofexhaust gases from the other end of the component to a downstreamexhaust pipe section after transmission thereof along said non-selectedpassages, the core zone having a radial size substantially equal to thebore of the upstream exhaust pipe section.